This invention relates to an array-type ultrasonic probe using a polymeric piezoelectric film as an ultrasonic transducer element.
Ultrasonic transducers have heretofore been widely used, for example, in depth sounders, fish sounders, and ultrasonic detectors. Recently, the application of ultrasonic transducers to medical diagnostic equipments has been rapidly developed. The ultrasonic transducer for medical diagnosis is operated on a principle that an ultrasonic wave generated by the ultrasonic probe is reflected at boundaries between portions of a living body having different acoustic impedances (velocity of sound.times.density), and the resultant ultrasonic echo is received by the ultrasonic probe and subjected to signal-conditioning to be displayed on a cathode-ray tube. In the ultrasonic wave generating part of such an ultrasonic probe, a vibrating member comprising a piezoelectric element is used. In order to improve the resolution of sectional plane images in a deep portion of a living body, a higher frequency of ultrasonic wave is gradually required. For complying with this trend, an array-type probe is preferred, wherein the piezoelectric element is divided into a number of small and thin unit elements. The array-type probes are generally classified, according to arrangement of unit piezoelectric elements, into those of the annular-type wherein fine unit elements having shapes of annular rings with gradually different diameters are radially arranged with a small gap therebetween, and those of the linear-type wherein linear or thin bar-shaped unit elements are arranged in parallel with each other with a small gap therebetween. Among them, the linear array-type ultrasonic probe (hereinafter merely referred to as "array-type ultrasonic probe") has an advantage that piezoelectric elements can be arranged at a high density per unit area of the ultrasonic transmitting and receiving face because of its simple arrangement, whereby sectional images along the transversal direction in addition to those along the depth direction can be obtained easily and at a high resolution by electronic scanning.
Conventionally, the arrangement structure of piezoelectric elements has been produced by applying a uniform plate or film of piezoelectric element on a substrate and cutting it with constant intervals to leave a plurality of piezoelectric elements separated from each other on the substrate. However, such a process wherein a piezoelectric element per se is subjected to cutting, is accompanied with several drawbacks such as deterioration of a piezoelectric element when a polymer piezoelectric element is used in order to comply with the requirement for a thin element, ill effects due to cutting dust and limitation in cutting accuracy. For this reason, there has been proposed an array-type ultrasonic probe having a structure as shown in FIGS. 1 through 4, wherein FIG. 1 is a perspective view, FIG. 2 is a plan view and FIGS. 3 and 4 show sections taken along the lines III--III and IV--IV, respectively, in FIG. 2 viewed in the directions of the arrows. Thus, in FIGS. 1 through 4, the probe comprises a substrate 1 having a top face 1a and a side wall 1b, and piezoelectric elements 2 arranged thereon and functionally separated from each other. These piezoelectric elements 2 have a laminar structure as shown in FIG. 3 which is a sectional view, i.e., comprising a substrate 1, and a plurality of reflection plates and back electrodes 2a separated from and in parallel with each other, a uniform or continuous piezoelectric film 2b such as a polarized film of a vinylidene fluoride resin and a uniform or continuous front electrode 2 c, successively applied onto the substrate in the order named. The front or surface electrode 2c is electrically connected through its extended portion 2cc to a lead wire 3 (FIG. 3), and a back electrode 2a is electrically connected to a lead wire 5 inserted through a bore 4 formed in the substrate 1 (FIG. 4). One preferable process for producing an array-type ultrasonic probe with a structure as described above has already been proposed by us (U.S. patent application Ser. No. 657,489).
We have found a difficulty with such an ultrasonic probe structure. The difficulty is one with respect to electrical connection between the back electrodes 2a and the lead wires 5. Thus, the connection structure is generally obtained, as shown in FIG. 5 corresponding to FIG. 4, by forming a stripe-form or bar-shaped reflection plate and back electrode 2a, exposing a lead wire 5 through a perforation 2aa formed near one end of the back electrode 5 and a bore 4 formed therebelow in the substrate 1, applying solder to join and fix the lead wire 5 and the back electrode 2a, and removing an excess of the solder by grinding. In order to obtain a high resolution, however, it is necessary to arrange, e.g., 1 mm-wide back electrodes 2a at equal gaps of the order of 0.02 to 0.1 mm. Accordingly, such small gaps can sometimes be filled with cutting dust, whereby required separation between the back electrodes 2a can be impaired.